A conventional solar cell structure with a p-type base has a negative electrode that is typically on the front-side or sun side of the cell and a positive electrode on the back. It is well known that radiation of an appropriate wavelength falling on a p-n junction of a semiconductor body serves as a source of external energy to generate electron-hole pairs in that body. The potential difference that exists at a p-n junction, causes holes and electrons to move across the junction in opposite directions, thereby giving rise to flow of an electric current that is capable of delivering power to an external circuit. Most solar cells are in the form of a silicon wafer that has been metallized, i.e., provided with metal contacts which are electrically conductive.
During the formation of a silicon solar cell, an aluminum paste is generally screen printed and dried on the back-side of the silicon wafer. The wafer is then fired at a temperature above the melting point of aluminum to form an aluminum-silicon melt, subsequently, during the cooling phase, an epitaxially grown layer of silicon is formed that is doped with aluminum. This layer is generally called the back surface field (BSF) layer, and helps to improve the energy conversion efficiency of the solar cell.
Most electric power-generating solar cells currently used are silicon solar cells. Process flow in mass production is generally aimed at achieving maximum simplification and minimizing manufacturing costs. Electrodes in particular are made by using a method such as screen printing from a metal paste.
An example of this method of production is described below in conjunction with FIG. 1. FIG. 1A shows a p-type silicon substrate, 10.
In FIG. 1B, an n-type diffusion layer, 20, of the reverse conductivity type is formed by the thermal diffusion of phosphorus (P) or the like. Phosphorus oxychloride (POCl3) is commonly used as the gaseous phosphorus diffusion source, other liquid sources are phosphoric acid and the like. In the absence of any particular modification, the diffusion layer, 20, is formed over the entire surface of the silicon substrate, 10. The p-n junction is formed where the concentration of the p-type dopant equals the concentration of the n-type dopant; conventional cells that have the p-n junction close to the sun side, have a junction depth between 0.05 and 0.5 μm.
After formation of this diffusion layer excess surface glass is removed from the rest of the surfaces by etching by an acid such as hydrofluoric acid.
Next, an antireflective coating (ARC), 30, is formed on the n-type diffusion layer, 20, to a thickness of between 0.05 and 0.1 μm in the manner shown in FIG. 1D by a process, such as, for example, plasma chemical vapor deposition (CVD).
As shown in FIG. 1E, a front-side silver paste (front electrode-forming silver paste), 500, for the front electrode is screen printed and then dried over the antireflective coating, 30. In addition, a back-side silver or silver/aluminum paste, 70, and an aluminum paste, 60, are then screen printed (or some other application method) and successively dried on the back-side of the substrate. Normally, the back-side silver or silver/aluminum paste is screen printed onto the silicon first as two parallel strips (busbars) or as rectangles (tabs) ready for soldering interconnection strings (presoldered copper ribbons), the aluminum paste is then printed in the bare areas with a slight overlap over the back-side silver or silver/aluminum. In some cases, the silver or silver/aluminum paste is printed after the aluminum paste has been printed. Firing is then typically carried out in a belt furnace for a period of 1 to 5 minutes with the wafer reaching a peak temperature in the range of 700 to 900° C. The front and back electrodes can be fired sequentially or cofired.
Consequently, as shown in FIG. 1F, molten aluminum from the paste dissolves the silicon during the firing process and then on cooling forms a eutectic layer that epitaxially grows from the silicon base, 10, forming a p+ layer, 40, containing a high concentration of aluminum dopant. This layer is generally called the back surface field (BSF) layer, and helps to improve the energy conversion efficiency of the solar cell. A thin layer of aluminum is generally present at the surface of this epitaxial layer.
The aluminum paste is transformed by firing from a dried state, 60, to an aluminum back electrode, 61. The back-side silver or silver/aluminum paste, 70, is fired at the same time, becoming a silver or silver/aluminum back electrode, 71. During firing, the boundary between the back-side aluminum and the back-side silver or silver/aluminum assumes an alloy state, and is connected electrically as well. The aluminum electrode accounts for most areas of the back electrode, owing in part to the need to form a p+ layer, 40. A silver or silver/aluminum back electrode is formed over portions of the back-side (often as 2 to 6 mm wide busbars) as an electrode for interconnecting solar cells by means of pre-soldered copper ribbon or the like. In addition, the front-side silver paste, 500, sinters and penetrates through the antireflective coating, 30, during firing, and is thereby able to electrically contact the n-type layer, 20. This type of process is generally called “firing through”. This fired through state is apparent in layer 501 of FIG. 1F.
Such aluminum pastes as mentioned above have been disclosed in many patent applications, for example, in US-A-2007/0079868. In the latter it is disclosed that the aluminum powder contained in the aluminum pastes may comprise atomized aluminum. The aluminum may be atomized in either air or inert atmosphere.
Silicon solar cells made from silicon wafers and having an aluminum back electrode are silicon/aluminum bimetallic strips and may exhibit a so-called bowing behavior. Bowing is undesirable in that it might lead to cracking and breaking of the solar cells. Bowing also causes problems with regard to processing of the silicon wafers. During processing the silicon wafers are generally lifted up making use of automated handling equipment using suction pads which may not work reliably in case of excessive bowing. Bowing requirements within the photovoltaic industry are typically <1.5 mm deflection of the solar cells. Overcoming the bowing phenomenon is a challenge especially with a view to silicon solar cells made from large and/or thin silicon wafers, for example, silicon wafers having a thickness below 180 μm, in particular, in the range of 120 to below 180 μm and an area in the range of above 250 to 400 cm2.